This invention relates to making thin strip and more particularly casting of thin strip by a twin roll caster.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of counter-rotating horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or tundish/distributor, from which it flows through a metal delivery nozzle located above the nip, which directs the molten metal to form a casting pool supported on the casting surfaces of the rolls above the nip. This casting pool is typically confined at the ends of the casting rolls by side plates or dams held in sliding engagement adjacent the ends of the casting rolls.
In casting thin strip by twin roll casting, the metal delivery nozzles receive molten metal from the movable tundish and deposit the molten metal in the casting pool in a desired flow pattern. Previously, various designs have been proposed for delivery nozzles involving a lower portion submerged in the casting pool during a casting campaign, and having side openings through which the molten metal is capable of flowing laterally into the casting pool outwardly toward the casting surfaces of the rolls. Examples of such metal delivery nozzles are disclosed in Japanese Patent No. 09-103855 and U.S. Pat. No. 6,012,508. In prior art metal delivery nozzles, there has been a tendency to produce thin cast strip that contains defects from uneven solidification at the chilled casting surfaces of the rolls.
The present invention provides an apparatus and method for continuous thin strip casting that is capable of substantially reducing and inhibiting such defects in the cast strip, and at the same time reducing wear in the delivery nozzles and the costs in thin strip casting. By testing, we have found that a major cause of such strip defects is thinning of the shells during casting. It is believed that the thinning of the shells is caused by localized high volume flow causing washing away of the shells during formation. Such thinning of the shells can result in ridges in the cast strip. We have found by changing the delivery nozzle that the flow of molten metal into the casting pool can be made more even and closer to uniform. This improved flow from the delivery nozzle into the casting pool is particularly notable in the region where the casting pool meets the casting surfaces of the rolls, generally known as the “meniscus” or “meniscus regions” of the casting pool and provides more even flow of molten metal.
In the past, the formation of pieces of solid metal known as “skulls” in the casting pool in the vicinity of the confining side plates or dams have been observed. The rate of heat loss from the casting pool is higher near the side dams (called the “triple point region”) due to conductive heat transfer through the side dams to the casting roll ends. This localized heat loss near the side dams has a tendency to form “skulls” of solid metal in that region, which can grow to a considerable size and fall between the casting rolls and causing defects in the cast strip. An increased flow of molten metal to these “triple point” regions, the regions near the side dams, have been provided by separate direct flows of molten metal to these triple point regions. Examples of such proposals may be seen in U.S. Pat. No. 4,694,887 and in U.S. Pat. No. 5,221,511. Increased heat input to these triple point regions has inhibited formation of skulls.
Australian Patent Application 60773/96 discloses a method and apparatus in which molten metal is delivered to the delivery nozzle in a trough closed at the bottom. Side openings are provided through which the molten metal flows laterally from the nozzle into a casting pool in the vicinity of the casting pool surface. The flow of molten metal into the casting pool was improved; however, unevenness in metal flow adjacent the casting roll surfaces caused washing away and thinning of the shells tending to cause defects in the cast strip. Further, there remained concern for wear on the delivery nozzle caused by the impact of the molten metal due to ferrostatic pressure, and turbulence caused as the molten metal moved through the delivery nozzle to discharge laterally into the casting pool below the meniscus of the casting pool. In addition, there was concern for extending the useful life of the delivery nozzles and in turn reducing the cost of producing thin cast strip.
The present invention provides an improved apparatus for casting metal strip and method of continuously casting metal strip. Disclosed is an apparatus for casting metal strip comprising:                (a) assembling a pair of casting rolls laterally disposed to form a nip between them,        (b) assembling an elongated metal delivery nozzle extending along and above the nip between the casting rolls, with at least one segment having a main portion and an end portion and an inner trough extending longitudinally through the main portion and into the end portion with end walls at opposite ends thereof, the inner trough communicating with outlets adjacent bottom portions formed in each segment adapted to deliver molten metal to a casting pool and the end portion having a reservoir portion having passages adapted to deliver molten metal to a casting pool,        (c) introducing molten metal through the elongated metal delivery nozzle to form a casting pool of molten metal supported on the casting rolls above the nip, such that molten metal is caused to flow into the inner trough of the delivery nozzle, from the inner trough through the outlets and through the reservoir portion passages into the casting pool, and        (d) counter rotating the casting rolls to deliver cast strip downwardly from the nip.        
The metal delivery nozzle may have an inner trough including a convex upper surface or, alternatively, a concave upper surface in the bottom portion of each segment.
The metal delivery nozzle may include an end portion having at least one longitudinally extending weir adjacent to the inner trough. The end portion may also include at least one reservoir adjacent the weir and opposite the inner trough.
The metal delivery nozzle may include an end portion having at least one reservoir extending laterally from the inner trough within the end portion.
The metal delivery nozzle may include outlets that are open at an end of the metal delivery nozzle.
Various aspects of the invention will be apparent from the following detailed description, drawings, and claims.